Astronomers have measured the background light from all the stars in the cosmos and inferred the number of stars created since the dawn of the universe, researchers announced Thursday.

This plot shows the locations of 150 blazars (green dots) used in the EBL study. The background map shows the entire sky and was constructed from four years of gamma rays with energies above 10 billion electron volts (GeV) detected by Fermi. The plane of our Milky Way galaxy runs along the middle of the plot. The Fermi LAT instrument is the first to detect more than 500 sources in this energy range. Credit: NASA/DOE/Fermi LAT Collaboration

As matter falls into a supermassive black hole, some of it is accelerated outward in the opposite direction. If this jet of energy is pointed at Earth, the Fermi telescope can detect its gamma rays.

Gamma rays are the most energetic form of light, sometimes having a billion times more energy than visible light.

Gamma rays from blazars travel billions of light-years to Earth, and like a lighthouse beam in dense fog, the brightness of gamma rays dims as the light journeys through the universe.

"We use blazars as cosmic lighthouses," said Marco Ajello, a researcher at the Kavli Institute for Particle Astrophysics and Cosmology at Stanford University and the Space Sciences Laboratory at the University of California at Berkeley.

The sum of the total starlight in the universe is called the extragalactic background light. As a gamma ray speeds through the cosmos, it can encounter the background light, which converts it into an electron and its antimatter counterpart, a positron, according to Justin Finke, an astrophysicist at the Naval Research Laboratory in Washington, D.C.

Scientists have dubbed the extragalactic background light as cosmic fog. Over greater distances, more gamma rays are absorbed by the cosmic fog.

"For blazars that are farther away, more of the gamma rays from that source will drop out from the spectrum," Finke said. "If the blazar is closer, we will observe more of the original gamma rays."

Researchers used 150 blazars to make their measurement of the cosmic fog, according to Ajello, lead scientist on the study, which was published in the Nov. 1 issue of Science Express.

The Fermi EBL measurements allow astronomers to constrain the number of stars that have ever shone in the universe. Moving forward, the measurements will serve as a guide for theoretical studies of the early universe, near the peak of star formation. Credit: NASA/Goddard Space Flight Center

Ajello said the study found the universe has an average of 1.4 stars per 100 billion cubic light-years, putting the average distance between stars at about 4,150 light-years.

"Fermi has used gamma rays to measure all the ultraviolet and visible light ever emitted by the stars of the universe," Ajello said.

Finke said the first stars formed when the universe was about 400 million years old. The rate of star production rose until the universe was about 3 billion years old, and is now steadily decreasing.

Launched in 2008, the $690 million Fermi telescope is designed to study the most energetic phenomena in the universe, including black holes, neutron stars, pulsars, and gamma ray bursts, the most powerful explosions in the cosmos.

Ajello said Fermi's discoveries act as a pathfinder for the James Webb Space Telescope, which will be able to image the first galaxies after the formation of the universe 13.7 billion years ago.

"With Fermi, we have the first step into this final cosmic frontier," said Volker Bromm, an associate professor at the University of Texas at Austin.

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